Lenard ray tube



Nov. 12, 1929.

c, M. SLACK LENARD RAY TUBE Filed April 25-: 28

INVENTOR G.M SLACK )1; 9*Q

ATTORNE be exceedingly thin.

Patented Nov. 12, 1929 UNITED STATES PATENT OFFICE CHARLES MORSE SLACK, OF BLOOMFIELD, NEW JERSEY ASSIGNOR TO "WESTINGHOUSE LAMP COMPANY,

A CORPORATION OF PENNSYLVANIA LENARD RAY TUBE Application filed April 23,

This invention relates to a Lenard ray tube of the type in which the electrons are projected at high velocities through a wall of the tube into the open air so as to be available for various purposes, such as effecting chemical reactions or for germicidal and sterilizing efiects.

Heretofore, Lenard ray tubes have been produced with metallic windows for the projection of an electron stream into the open air. Lenard as early as 1891 provided a vacuum tube with a small window of aluminum ioil of extreme thinness against which cathode rays were shot at a sufiiciently high velocity to pass completely therethrough and to excite phosphorescence in the air a few millimeters away. Due to the low velocity of the electrons and the small quantity thereof obtainable from this tube, is remained only a laboratory experiment. Most recently, however, improvements have been made in the Lenard ray tube among others, by O. Eisenhut (Heidelberg Dissertations May 1921),

.Kruger and Utesch (Ann. d. Phys. 78, 1925,

pp. 113-156) and W. D. Coolidge (Br. Patent 251,635). In this latter patent, there is described a Lenard ray tube of the hot cathode type, provided with a metallic window but capable of operation at several hundred thousand volts, whereby the penetrating pow: er of the cathode rays in the open air is increased many fold.

The metallic windows through which the cathode rays are projected in these tubes must Coolidge in his British patent specifies molybdenum foil of a thickness of .0003 inches and Lenard employed a window of aluminum foil of only .0001 inches thickness.

Metal of this degree of thinness is extremely fragile and if of any appreciable area, it must be supported by a heavier metallic structure to which it is necessary to secure the foil in a gas tight manner. This metallic structure, in turn, must be sealed to the glass portion of the envelope also in a gas tight manner.

enced in producing a metal window of this nature and the process is expensive and 1n- Considerable difiiculty has been experi-.

1928. Serial No. 272,194.

volved, rendering the devices costly and retarding their use in many fields where the beneficial effects of the cathode rays could be utilized. Due to the large mass of metal of which the anode is formed, complete degasification of the tube is extremely difficult and it has been necessary to maintain a pumping connection for the device or to provide a gas absorbing tube containing charcoal which, during operation is immersed in liquid air.

One of the objects of the present invention is to overcome the difficulties inherent in the use of metal windows and to employ a portion of the glass envelope of the device as the window ghrough which the cathode rays are proecte Another object is to produce a novel window for a Lenard ray tube in which there will be a low loss of energy of the cathode ray in passing through the window.

A still further object is to produce a window for a Lenard ray or ion tube which is of low density and which may be made extremely thin while retaining'sufficient strength to resist the external atmospheric pressure.

Other objects and advantages will hereinafter appear.

The loss of energy of cathode rays in passing through a material increases with and is proportional to the square root density of the material and the square root thickness thereof. In order to obtain the requisite thinness of the window with suflicient mechanical strength to withstand the differential of pressure between the interior and exterior of the envelope, it has been thought necessary, heretofore, to employ metal windows. Molybdenum has been considered the most practical metal for this service due to its ductility and high elastic limit. The density of molybdenum is rather high, however. so that the energy which is lost in passage of the cathode rays through the window is relatively large. Aluminum has a much lower density and the energy loss therein is much less than that for molybdenum, but its strength is low and it is necessary to employ heavier foil. 'Aluminum, moreover, does not seal readily with glass or other metals and increases the difiiculty of producing gas tight conditions in the tube.

I have discovered, however, that the oathode rays can be transmitted directly through the glass wall of the device. Ordinary glass as used for the envelope of high voltage devices, such as X-ray tubes, cathode ray tubes, etc., is ordinarily about 1 mm. in thickness in order to give the requisite strength to resist handling and support of the heavy electrode structure. At the ordinary voltage available for X-ray tubes, that is at potentials up to several hundred thousand volts, this thick- 'ness of glass is opaque to cathode rays.

I have found, however, that certain por tions of the envelope may be made of extreme thinness, as low as .00025 inches if properly shaped and protected, While still retaining sufficient strength to resist atmospheric pressure. Small windows may be made as thin as 0.0001 inches. A window which has been thinned to this extent will transmit cathode rays with a very low energy loss, since at a given voltage the cathode rays will penetrate about three times the thickness of glass as of molybdenum, due to the difference in density thereof.

The absorption of energy by the envelope or window of the tube varies with the voltage decreasing as the voltage increases. Thus for instance, with a glass window of about 0.00025 inches thickness, it requires a potential of about 26,000 volts.

between the anode and cathode to cause most of the cathode rays to penetrate the window. At this voltage, all of the energy of the electron is absorbed by the glass which, as a consequence, rapidly heats up. As the voltage applied between the electrodes in creases, however,the loss of energy in the glass window decreases very rapidly. With a velocity of the electron corresponding to 60,000 volts, the 0.00025 inch glass window will transmit the electrons with an emergent velocity corresponding to 54,000 volts, there being a loss of velocity in the window corresponding only to about 6,000 volts. Thus for instance, the same thickness of glass will absorb only about 3,000 volts with an impressed potential of 100,000 volts. This decrease in energy loss in the glass, at higher voltages, effects a corresponding decrease in the heating effect of the glass and contrary to expectation, it is possible to continuously subject this window to cathode rays at these higher voltages for considerable periods of time without destruction thereof.

In order to maintain the heating eiiectof the cathode rays in passing through the glass, to a low value, it is essential that they strike the glass substantially normal to its surface, that is, at an angle of not more than degrees from normal, since the rate of energy *bsorption by the glass increases rapidly with he thickness. For instance, with a thickness of 0.001 inches the critical voltage required to just pass through the glass is about 52,000 volts.

Since the energy loss in the glass decrease at the higher voltages, it is possible to employ thicker windows in the higher voltage,

tubes without materially increasing the energy loss in the glass. Thus, for a thickness of 0.001 inches the energy absorption in the glass at potentials of 100,000 volts between the electrodes corresponds to a decrease in inter-electrode potential of only about 16,000 volts and at 200,000 volts there is a loss of energy corresponding only to a decrease of about 8,000 volts between the electrodes.

The window through which the cathode rays are transmitted may be made by drawing in or blowing out a restricted portion of the envelope to form a bulbous portion. The drawn-in window may be made thinner for the same strength, than the blown-out window since in the former case, the atmospheric pressure exerts a tensional instead of a bending or buckling strain force. This drawn-in bulb has the further advantage of being protected by the heavy surrounding portion of the envelope. The chief advantage of the blown-out window is that it enables the window to be brought closer to the material to be treated.

The cathode rays are drawn over from the cathode by an anode maintained at a high potential and having an aperture through which the electrons pass at high velocity. This aperture is disposed opposite the bulbous window in such manner and should be of such size that the electrons emerging therefrom strike the window substantially normal to the surface thereof.

If desired, the interior surface of the window may be coated with an exceedingly thin metallic film electrically connected to the anode, so as to prevent the accumulation of a negative charge on the window and the danger of discharges between the glass window and the highly positively charged anode. This, however, in most cases is un necessary as the intense ionization produced by the rays allows this charge to leak off.

The cathode stream after passing through the window, is dispersed by collison with gas molecules in the air. These deflected electrons may bombard the outside surface of the bulbous window, in the case of the drawn-in Window, causing undesirable heating and danger of destruction of the window if the operation of the tube is prolonged. This heating may be reduced by coating the exterior of the side walls of the bulbous window with a protective material of such thickness that the electrons will not readily penetrate therethrough to the glass. A coating of shellac may be applied to the glass for this purpose. This shielding coating serves also device, as at 12.

to mechanically strengthen the extremely thin bulb of glass.

In order that the invention may be more fully understood, reference will be had to the accompanying drawing in which:

Fig. 1 is a side view, partly in section, of a Lenard ray tube embodying my invention;

Fig. 2 is a fragmentary View similar to Figure 1 showing modified form of anode construction; and

Fig. 3 is a fragmentary view showing a modified form of window.

The tube shown in Figure 1 comprises a glass envelope 1 having a relatively heavy wall and containing a filamentary cathode 2 and an anode 3. The cathode end 4 of the tube has a reentrant glass stem 5 termiiiating in a press 6 through which the leading-in conductors 7 for the cathode, are sealed.

The envelope may be composed of any suitable glass, such as pyrex or lime glass, but I prefer to use a glass more opaque to X-rays which may be generated in the tube, such as lead glass or a boro-silicate glass known in the trade as 702-1. The lead wires in the case of 702-P glass may be of either tungsten or molybdenum.

The cathode 2 may be of any suitable electron emitting material, but preferably I construct it of tungsten or tantalum in the form of a coil mounted within a focusing cup 8, as is usual in X-ray tube construction.

An electro-static' shield 9 surrounds the cathode and protects the seal from puncturing. It also prevents sharp point sparking from the cathode. This shield may take the form of a split metal tube of nickel, Monel metal, chrome-iron of other metal, held in place on the reentrant stem 5 by friction. Obviously, other convenient methods of support may be readily devised.

The anode consists of a tube 10 preferably of copper and is provided with a leading-in conductor 11 sealed through the wall of the The tubular anode is sup ported in the envelope by a split collar 13 of chrome-iron or other suitable metal which fits snugly into the glass envelope and is secured to the anode in any suitable manner as by pins, screws, friction, soldering, etc. The tubular anode is arranged with its axis aligned with the axis of the electron stream from the cathode so that the electrons drawn over by the anode pass there through and are projected against the window 14. The tubular anode also serves as a shield to prevent the electrons striking undesired portions of the envelope. For this purpose, the end 15 adjacent the cathode may be enlarged.

The end 16 of the envelope adjacent the anode may be reduced in diameter where the window 14 is formed therein. Preferably the window 14 is formed in the closed end of a short length of tubing which is subsequently sealed to the reduced portion of the envelope.v

In forming the window, it is only necessary to heat the closed end of a piece of tubing, as by a blow pipe flame, and to create a suction on the interior thereof so that the plastic glass is drawn in, the suction and heating being continued until the requisite size and thickness of the bulbous portion is produced. The window 14 may preferably have a thickness of from 0.0001 to 0.005 inches. Since the center of the closed endof the tube is heated to the highest temperature it becomes the thinnest part of the bulb, the walls gradually increasing in thickness from such central section to the outer edge of the bulbous portion where it joins the end of the tubular part 16 of the envelope. This strengthens the bulbous end of the envelope while enabling the central portion through which the cathode rays are projected, to be very thin.

The bulbous window may be further thinned to the desired thickness by etching with a weak solution of hydrofluoric acid. A convenient method of determining the thickness of the window is by the voltage required to cause 'cathrode rays to pass through the glass window. Thus, if one side of the window is coated with a material which is fluoresced by cathode rays and the opposite side thereof is placed opposite the window of a cathode ray tube, the voltage required between the electrode of the tube in order to cause a florescence of the coating on the window is a measure of the thickness of the glass.

The outer end 17 of the envelope may be enlarged in order to accommodate a bulbous portion 14 of a diameter somewhat larger than the opening 18 in the tubular anode 3, so that all portions of the window opposite the opening 18 will be substantially normal to the direction of travel of the cathode rays and therefore, so that the thickness of glass traversed by the electrons will be substantially uniform. If the electrons were permitted to strike the glass obliquely to the surface of the bulbous portion, the energy loss in the greater thickness of glass encountered would cause undesirable heating with danger of causing destruction of the window or limiting the period of operation of the tube.

The windows 14 should preferaby be composed of a strong, heat resisting glass of low density such as pyrex glass, although other glasses, quartz or non-porous porcelain may be used, and by the term vitreous material, it is intended to include all materials of this nature.

In Figure 2, a modification is shown in which the tubular anode 3 is replaced by a cup shaped anode 19 having an aperture 20 through which the cathode rays emerge. The anode is supported by a sleeve 21 engaging the inner wall of the envelope. The inner surface of the window 14 may be coated with an exceedingly thin film of conductive material 22 which is in electrical connection with the anode 19, whereby the accumulation of a negative charge on the window due to the electrons striking the same, is prevented. Such negative charge if allowed to accumulate may cause a discharge to the anode and consequent destruction of the window.

The exterior of the bulbous portion 14, with the exception of the inner end through which the electrons pass, is coated with a strengthening and shielding material 23 to prevent stray electrons deflected thereagainst, from unduly heating the glass.

In Figure 3, the bulbous window 24 is formed externally of the side walls of the en- Velope. This form, while not as strong as the drawn-in window, is satisfactory for high voltage tubes in which heavier windows may be employed. It has the advantage that the window may be brought closer to the material being treated and is subject to less bombardment by deflection of electrons in the air.

A tubular shield 25 surrounds the window and protects the same from mechanical shock. This shield is shown composed of glass but may be of any other suitable material, as metal, fiber, bakelite, etc.

The electrodes should be thoroughly degasified in accordance with the practice followed in the construction of X-ray tubes. Briefly, such degasification may be carried out by heating the metal parts to a red heat in a vacuum furnace prior to assembly in the envelope to effect a removal of the major portion of the occluded gases. After assembly, the electrodes and associated parts are sealed into the tube, a high vacuum is created therein and a final degasification of the electrodes effected by electronic bombardment and high frequency induction heating while the tube is on the exhaust pumps. After this final degasification, the tube may be sealed off from the pumps and due to the relatively small mass of metal therein, such vacuum is maintained without the use of gas absorbing ma.- terials within the tube and liquid air.

If desired, a small pressure of from 1 to 10 microns of monatomic gas such as neon may be admitted to the envelope.

The invention has been described specifically in connection with a Lenard ray or negative ion'tube, but it is to be understood that the window construction and other features thereof are applicable to the transmission of .both negative ions and positive ions and it is to be understood that the invention is not limited to the exact construction shown but various modifications and changes may be made therein without departing from the essential attributes of the invention as defined in the appended claims.

What is claimed is:

1. A Lenard ray tube operable in the absence of appreciable gas ionization within the tube to project cathode rays into the atmosphere comprising an envelope, a source of electrons therein, an anode having an aperture therein, and a window transparent to cathode rays disposed opposite said aperture, said window being composed of vitreous material.

2. A Lenard ray tube comprising an envelope, a source of electrons therein, a bulbous window in said envelope composed of vitreous material and an anode for projecting a stream of high velocity electrons against said window, said window having a thickness of from 0.0001 to 0.005 inches.

3. A Lenard ray tube operable in the absence of appreciable gas ionization within the tube to project cathode rays into the atmosphere comprising an envelope having a vitreous, portion, a source of electrons within said envelope, a vitreous Window in said vitreous portion of a thickness of from 0.0001 to 0.005 inches, an anode for directing a stream of high velocity electrons against said window, and means for preventing the accumulation of a negative charge on said vitreous window.

" 4. A Lenard ray tube comprising an envelope, a bulbous window therein having a central dome portion of a thickness of not over a few thousandths of an inch, a source of electrons within the envelope, an anode disposed between said window and said source of elec' trons, said anode having an aperture therein through which electrons are directed against said window, and shielding means on the exterior of a portion of said bulbous window to protect the same from stray electrons.

5."A Lenard ray tube operable in the absence of appreciable gas ionization within the tube, comprising an envelope, an electron emitting cathode extending from one end thereof, a vitreous bulbous shaped window at the opposite end of the envelope permeable to high speed electrons and an anode for directing cathode rays upon the window at a sufiiciently high rate to pass through the same.

6. A Lenard ray tube comprising an envelope, an electron emitting cathode ex.- tending from one end thereof, a vitreous bulbous shaped window at the opposite end of the envelope permeablev to high speed electrons, an anode for directing cathode rays upon the window at a sufiiciently high rate to pass through the same and means surround ing said bulbous window for shielding the same from mechanical shock.

7 A window for an ion tube permeable to high velocityions comprising a bulbous body of low density glass having at the dome portion a thickness of from 0.0001 to 0.005' inches.

8. A Lenard ray tube comprising an envelope having a vitreous wall and containing an electron emitting cathode therein and an anode, for directing a stream of electrons at high velocity against the vitreous wall of said envelope, said wall at the region of impact of said'electrons having a thickness of from about one ten thousandth to five thousandths of an inch.

9. A window permeable to high velocity ions comprising a bulbous body of vitreous material having at the dome portions a thickness of from about 0.0001 to 0.005 inches.

10. A window permeable to high velocity ions comprising a substantially bulbous body of relativel heavy glass having a concentric re-entrant bulbous portion, sald re-entrant portion having at the center a thickness of rom about 0.0001 to 0.005 inches.

11. In an ion tube, a window permeable to high velocity ions, said window being composed of vitreous material of a thickness of from 0.0001 to 0.005 inches.

12. A Lenard ray tube comprising an envelope, a bulbous window therein having a central dome portion of a thickness of from 0.0001 to 0.005 inches, means within said envelope for projecting a stream of high velocity electrons against said window and means for confining said stream of electrons substantially to said dome portion.

13. A Lenard ray tube operable in the absence of appreciable gas ionization within the tube to project cathode rays into the atmosphere comprising an envelope, 9. source of electrons within said envelope, a vitreous window in said envelope of a thickness of from 0.0001 to 0.005 inches, an anode for directing a stream of high velocity electrons against said window and a thin conducting coating on one side of said window for preventing the accumulation of a negative charge thereon.

14. A Lenard ray tube operable in the 'absence of appreciable gas ionization within the tube to project cathode rays into the atmosphere comprising an enve ope, a source of electrons within said envelope, a vitreous window in said envelope of a thickness of from 0.0001 to 0.005 inches, an anode for directing a stream of electrons against said window and a thin conductive coating on one side of said window in electrical connection with said anode,

In testimony whereof, I have hereunto subscribed my name this 20th day of A ril 1928.

CHARLES MORSE .S CK. 

